Online auctions are becoming an increasingly important channel for electronic commerce. There exist more than 150 online auction sites on the Internet. It is hard for users to attend, monitor, and bid in the multiple auction sites simultaneously. In this paper, we propose BiddingBot, which is a multi agent system that supports users to attend, monitor, and bid in multiple auctions. BiddingBot monitors prices of goods in several online auction sites to get reasonable market prices of goods, and uses a new cooperative bidding mechanism to effectively bid in auctions. The features of BiddingBot are described as follows: (a) the system enables users to bid effectively in the multiple auctions. (b) Users can find a reasonable market price of goods and can avoid “winner's course” by using the system. (c) The system enables programmers to extend its functions incrementally to manage new auction sites.